Crystalline polyester-based resins such as polyethylene telephthalate (PET), glycol-modified PET (PET-G) and polyethylene naphthalate (PEN) are used in various fields. For example, in the production of IC cards, a PET sheet is laminated on the outer layer of a sheet-like circuit substrate for an IC card, which is formed of an insulating resin on which electronic parts are mounted, to protect an IC card or maintain the form thereof. However, among plastics, a crystalline polyester-based resin such as PET has extremely low polarity, and thus when one tries to attach it by using an adhesive comprising a solvent, sufficient adhesion strength cannot be obtained.
Therefore, for example, Patent Document 1 discloses use of a hot melt-type sheet-like adhesive for attaching a PET sheet on the outer layer of a circuit substrate for an IC card by lamination. According to the hot melt-type sheet-like adhesive as disclosed in Patent Document 1, a base material formed of a crystalline polyester-based resin can be attached strongly, whereby an adhesive having excellent chemical resistance can be provided. However, the production of a hot melt-type sheet-like adhesive requires a step of forming an adhesive into a sheet-like shape by extrusion molding or the like.
Furthermore, a sheet-like adhesive has drawbacks that a certain thickness is required for the production of a sheet and thus it cannot be used for various sheet thicknesses. Therefore, an adhesive having a high degree of freedom in design of the thickness of an IC card is required.
Accordingly, in order to solve the problems, the applicant of the present patent application has previously proposed a heat curable adhesive characterized by comprising (a) a non-crystalline polyester resin containing a hydroxyl group, (b) a multifunctional polyisocyanate, (c) a solvent which dissolves the non-crystalline polyester resin, and (d) an agent for imparting thixotropy (Patent Document 2). The present invention can provide an adhesive having excellent chemical resistance. However, the invention had a problem that increase in thickening ratio during storage was high and storage stability was poor.